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Chem 10
Atoms and Elements: Chapter 4
ELEMENTS AND ATOMS
All known substances on Earth and probably the universe are formed by combinations of more than 100
elements.
An element is a fundamental or elementary substance that cannot be broken down into simpler substances by
chemical means.
Each element has a number.
Beginning with hydrogen as 1, the elements are numbered in order of increasing complexity.
Most substances can be decomposed into two or more simpler substances.
Water can be decomposed into hydrogen and oxygen.
Table salt can be decomposed into sodium and chlorine.
An element cannot be decomposed into a simpler substance.
An atom is the smallest particle of an element that can exist.
An atom is the smallest unit of an element that can enter into a chemical reaction.
An atom is very small
The diameter of an atom is 0.1 to 0.5 nm. This is 1 to 5 ten billionths of a meter.
Even smaller particles than atoms exist. These are called subatomic particles.
Dalton’s Atomic Theory
1. Elements are composed of minute indivisible particles called atoms.
2. Atoms of the same element are alike in mass and size. Atoms of different elements have different masses
and sizes. (Is this still correct?)
3. Chemical compounds are formed by the union of two or atoms of different elements in whole number
ratios.
WHAT’S IN AN ATOM? DISCOVERING SUBATOMIC PARTICLES!
In 1897 Sir Joseph Thompson demonstrated that cathode rays:
- travel in straight lines
- are negative in charge
- are deflected by electric and magnetic fields
- produce sharp shadows
- are capable of moving a small paddle wheel
Subatomic Particles: Electron
This was the discovery of the fundamental unit of charge
– the electron.
Subatomic Particles: Protons
Eugen Goldstein, a German physicist, first observed protons in 1886:
Thompson determined the proton’s characteristics.
Thompson showed that atoms contained both positive and negative charges.
This disproved the Dalton model of the atom which held that atoms were indivisible.
But the mass of the atom could not be accounted for by the mass of protons inside it. There had to be
something else.
Subatomic Particles: Protons
James Chadwick discovered the neutron in 1932.
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Its actual mass is slightly greater than the mass of a proton.
The Nuclear Atom
Radioactivity was discovered by Becquerel in 1896.
Radioactive elements spontaneously emit alpha (
their nuclei.
consisting of 2 protons and 2 neutrons.
The Rutherford Experiment
In 1911, Ruther
Most of the particles passed through the foil with little or no deflection.
k.
(See Figure 4.4 in text.)
Rutherford knew that like charges repel.
Rutherford concluded that each gold atom contained a positively charged mass that occupied a tiny volume.
He called this mass the nucleus.
mostly empty space.
the nucleus was very heavy and dense.
(See Figure 4.5 in text.)
General Arrangement of Subatomic Particles
Rutherford’s experiment showed that an atom had a dense, positively charged nucleus.
Chadwick’s work in 1932 demonstrated the atom contains neutrons.
Rutherford also noted that light, negatively charged electrons were present in an atom and offset the positive
nuclear charge.
Rutherford put forward a model of the atom in which a dense, positively charged nucleus is located at the
atom’s center.
The negative electrons surround the nucleus.
The nucleus contains protons and neutrons
ATOMS AND THEIR SUBATOMIC PARTICLES
The atomic number of an element is equal to the number of protons in the nucleus of that element.
The atomic number of an atom determines which element the atom is.
Every atom with an atomic number of 1 is a hydrogen atom.
Every carbon atom contains 6 protons in its nucleus.
1H
6C
Ions: atoms that have lost or gained electrons
Positive ions were explained by assuming that a neutral atom loses electrons.
Negative ions were explained by assuming that extra electrons can be added to atoms.
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When one or more electrons are lost from an atom, a cation is formed.
When one or more electrons are added to a neutral atom, an anion is formed.
Isotopes of the Elements
Atoms of the same element can have different masses.
They always have the same number of protons, but they can have different numbers of neutrons in their nuclei.
The difference in the number of neutrons accounts for the difference in mass.
These are isotopes of the same element.
Isotopic Notation
Carbon-12 is 126C.
Carbon-14 is 146C.
Examples of Isotopes
Element
Hydrogen
Hydrogen-2
Hydrogen-3
Protons
1
1
1
Electrons
1
1
1
Neutrons
0
1
2
143
146
Uranium-235
Uranium-238
92
92
92
92
Chlorine-35
Chlorine-37
17
17
17
17
18
20
Symbol
1
1H (Protium)
2
1H (Deuterium)
3
1H (Tritium)
235
92U
238
92U
35
17Cl
37
17Cl
Atomic Mass
The mass of a single atom is too small to measure on a balance.
Using a mass spectrometer, the mass of the hydrogen atom was determined.
The mass of one hydrogen atom was determined to be 1.673 x 10-24 g.
NOTE: Drawing of Modern Mass Spectrometer
A typical reading from a mass spectrometer. The two principal isotopes of copper are shown with the
abundance (%) given.
ATOMIC MASS UNITS AND RELATIVE ATOMIC MASS
Single atomic masses are too small to weigh on a balance. To overcome this problem a system of relative
atomic masses using “atomic mass units” was devised to express the masses of elements using simple
numbers.
The standard to which the masses of all other atoms are compared to was chosen to be the most abundant
isotope of carbon.
A mass of exactly 12 atomic mass units (amu) was assigned to the carbon-12 atom. An amu is defined as
exactly equal to the mass of a carbon-12 atom. 1 amu = 1.6606 x 10-24 g
Isotopes of the same element have different masses.
The listed atomic mass of an element is the average relative mass of the isotopes of that element compared to
the mass of carbon-12.
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Calculating relative atomic mass:
Convert percent abundance to fraction by dividing by 100
Multiply fraction abundance times each isotopes mass
Sum up the results
Rel Atomic Mass = f1*m1 + f2*m2 + …
See example on next slide
To calculate the atomic mass multiply the atomic mass of each isotope by its fractional abundance and add the
results.
(Given that copper has the two isotopes shown in the mass spectrometer above, with 69.09% being copper 63
with an isotopic mass of 62.9298 amu and 30.91% copper 65 at 74.9278 ams, then:
(62.9998 amu)0.6909 = 43.48 amu
(74.9278 amu)0.3091 = 20.07 amu
63.55 amu
Relationship between Mass Number and Atomic Mass
Atomic mass found on the Periodic Table is RELATIVE atomic mass, the weighted average of all the
isotopes.
Each isotope of an element has a unique WHOLE number called the mass number, the sum of protons and
neutrons. Mass number is NOT found on most Periodic Tables.
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ATOMIC MASS IS NOT THE SAME AS MASS NUMBER
ATOMIC MASS IS NOT THE SAME AS MASS NUMBER
ATOMIC MASS IS NOT THE SAME AS MASS NUMBER
******************************************************************
REMEMBER: ATOMIC MASS IS AN AVERAGE OF THE DIFFERENT ISOTOPES’ MASSES. It will
have decimal points!
Atomic Mass, Mass Number & Abundance of Isotopes
EXAMPLE: Chlorine has two isotopes, chlorine-35 and chlorine-37. Write down the atomic number, mass
number and number of neutrons for each. Then look up the atomic mass. Which isotope is in greater
abundance?
Cl-35: Z = 17, A = 35, 35 - 17 = 18 neutrons
Cl-37: Z = 17, A = 37, 37 - 17 = 20 neutrons
Atomic mass is 35.453 amu.
Closer to 35, therefore chlorine-35 is more abundant.
Now do problem 105 in your textbook.
(Show work here.)
Relationship Between Mass Number and Atomic Number
The mass number minus the atomic number equals the number of neutrons in the nucles.
For silver-109, the mass number is 109, the atomic number is 47; 109-47 = 62 neutrons.
Elements are not distributed equally by nature.
Oxygen is the most abundant element in the human body (65%).
Oxygen is the most abundant element in the crust of the earth (49.2%).
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In the universe, the most abundant element is hydrogen (91%) and the second most abundant element is
helium (8.75%).
(Sources of element names: not required to know, just interesting.)
Table: Symbols of the Most Common Elements
Table: Symbols of the Elements Derived from Early Names
Element Names and Symbols
Element names and symbols are in Periodic Table inside front cover.
Symbols: Learn first 36 on Periodic Table, plus Ag, Au, Pt, Hg, Sn, Pb, I, U, Ba, and Rn.
Also memorize that some elements exist as diatomic molecules : H2, O2, N2, F2, Cl2, Br2, and I2
The Periodic Table of the Elements
The periodic table was designed by Dimitri Mendelev in 1869.
In the table each element’s symbol is placed inside of a box.
Above the symbol of the element is its atomic number.
The elements are arranged in order of increasing atomic number.
Elements with similar chemical properties are organized in columns called families or groups .
(List of noble gases.)
18 columns - called groups
7 rows - called periods
Representative elements are in groups 1,2, 3A-8A (13-18)
Transition metals are in groups 3-12
Groups of elements have similar chemical properties
SPECIAL GROUP NAMES:
1A (1) = alkali metals
2A (2) = alkaline earth metals
8A (18) = noble gases (six)
7A (17) = halogens (four)
NOTE: H really belongs to its own group, which is why it’s shown by itself in my periodic table! It has 1
electron in outer shell, like the 1A elements, but it’s not a metal, and reacts more like group 7A.
The Periodic Table of Elements (from your textbook; also see large handout)
Most elements are metals. (List of physical properties of metals here.)
Chemical properties of metals:
Metals have little tendency to combine with each other to form compounds.
Many metals readily combine with nonmetals to form ionic compounds.
They can combine with sulfur, chlorine and oxygen.
A few of the less reactive metals such as copper, silver and gold are found in the free state.
Metals can mix with each other to form alloys.
Brass is a mixture of copper and zinc.
Bronze is a mixture of copper and tin.
Steel is a mixture of carbon and iron.
Physical Properties of Nonmetals
Everything metals are, nonmetals are NOT!
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Lack luster (they are dull)
Have relatively low melting points
Have low densities.
Poor conductors of heat and electricity
At room temperature, carbon, phosphorous, sulfur, selenium, and iodine are solids.
Physical State at Room Temperature (of the Nonmetals)
The Metalloids
Metalloids have properties that are intermediate between metals and nonmetals: they include
boron, silicon, germanium, arsenic, antimony, tellurium, polonium
Important Groups - Hydrogen
Nonmetallic
Colorless, diatomic gas – exists as H2
Very low melting point & density
Reacts with nonmetals to form molecular compounds
HCl is acidic gas
H2O is a liquid
Reacts with metals to form hydrides
metal hydrides react with water to form H2
HX dissolves in water to form acids
Important Groups – IA, Alkali Metals
Hydrogen usually placed here, though it doesn’t belong
Soft, low melting points,low density
Very reactive, never find uncombined in nature
Tend to form water soluble compounds withcolorless solutions
React with water to form basic (alkaline) solutions and H2: 2 Na + 2 H2
releases a lot of heat
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Important Groups – IIA, Alkali Earth Metals
Harder, higher melting, and denser than alkali metals
-green
Reactive, but less than corresponding alkali metal
Form stable, insoluble oxides from which they are normally extracted
Oxides are basic = alkaline earth
Reactivity with water to form H2
Important Groups – VIIA, Halogens
Nonmetals
F2 & Cl2 gases; Br2 liquid; I2 solid
All diatomic
Very reactive
Cl2, Br2 react slowly with water: Br2 + H2
React with metals to form ionic compounds
HX all acids
HF weak < HCl < HBr < HI
Important Groups – VIIIA, Noble Gases
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All gases at room temperature
Very low melting and boiling points
Very unreactive, practically inert
Very hard to remove electron from or give an electron to
Review
What is the atomic number of boron, B?
What is the atomic mass of silicon, Si?
How many protons does a chlorine atom have?
How many electrons does a neutral neon atom have?
Will an atom with 6 protons, 6 neutrons and 6 electrons be electrically neutral?
Will an atom with 27 protons, 32 neutrons and 27 electrons be electrically neutral?
Will a Na atom with 10 electrons be electrically neutral?
What is the mass number of fluorine?
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